Heating systems and air conditioning systems are generally operable in response to a thermostat. Thermostats open or close in response to the surrounding temperature and act to maintain the heating and air conditioning system in operation until a preset temperature is reached. Both heating and air conditioning systems operate efficiently only for a short period of time. In a furnace, the temperature of the heat exchanger increases as fuel burns. Eventually, the heat exchanger surface in contact with the fire box becomes so hot that the heated air is forced from the fire box through the chimney without raising the temperature of the heat exchanger and thus energy is wasted.
For this reason, it is preferable to periodically shut down the burner while air is forced through the heat exchanger causing the surface to cool down. This permits the heat from the fire box to efficiently transfer heat to the heat exchanger.
With an air conditioning system, the compressor operates and causes coils within the plenum to cool to a minimum of 34.degree. C. Even though the coils are at their minimum temperature, the compressor continues to operate using energy, but without value.
Various duty cycling switches have been employed in the past such as those disclosed in Kinsey U.S. Pat. No. 3,136,730, Hamilton U.S. Pat. No. 3,921,899, Brown U.S. Pat. No. 4,534,181. These switches generally detect temperature within the plenum. This temperature changes extremely quickly and responds to minor changes. Thus, it is not a reliable way to control a furnace or air conditioner.
A duty cycling control switch disclosed in Davis et al U.S. Pat. No. 4,470,267 is specifically designed to retrofit existing units. It is designed to mount to the exterior wall of a furnace or air conditioning plenum and opens and closes in response to changing temperatures of the plenum wall and heating system. The switch can be adjusted to deactivate under desired conditions. More particularly, the switch is a bimetal duty cycling switch which is enclosed within a case wherein the base of the case is metal. The bimetal switch is in thermal contact with the metal base plate which in turn contacts the plenum wall. This switch, when mounted to the plenum, has been found to efficiently duty cycle air-conditioners and furnaces.
Unfortunately, the plenum is not always accessible. Therefore, in these situations the switch must be mounted on a heat duct which is accessible. Much newer construction has eliminated metal duct work. This has been replaced with flexible cylindrical duct work which is formed from plastic reinforced with a spiral wire. Since this is plastic, which is a poor heat conductor, one cannot attach a duty cycling switch to its exterior and accurately control the furnace or air conditioner. Until now if the plenum was not accessible and the ducts were flexible plastic, one could not attach a heat sensitive duty cycling switch to the HVAC unit.